FIG. 1 is a cross-sectional view showing a structure of a coreless crawler 1 having such a structure as follows:
Main cords 2 are embedded in rows in a longitudinal direction of the crawler 1. Two layers of bias cords (a, b) are embedded at an outer peripheral side of the main cords 2, and bias cords (c) are embedded at an inner peripheral side of the main cords 2. Rubber protrusions 6 are formed at constant intervals at a central portion on an inner peripheral surface of the crawler 1, and are subject to driving forces from sprockets. Rubber lugs 7 are formed on an outer peripheral surface of the crawler 1.
In molding a crawler, the bias cords (a, b, c) and the main cords 2 are not simply disposed as they are in a mold. Instead, they are held from above and bottom between unvulcanized rubber sheets, i.e., so-called “treatment rubbers”, and formed into a belt shape. Then, they are stacked in the mold to mold a crawler.
The bias cords (a, b) form inverse angles at about 35 degrees with respect to the main cords 2, and the bias cords (c) are laid approximately perpendicular to the main cords 2. By these cords, lateral rigidity of the crawler is improved, rubber cracking due to an outer force is prevented, and rubber cracking is halted at positions of the bias cords even if rubber cracking occurs to thereby prevent rubber cracking from affecting the main cords.
However, since inner peripheral surfaces 1A and 1B in left and right width directions of the rubber protrusions 6 are rolling surfaces of track rollers, the inner peripheral surfaces 1A and 1B are subject to repetitive compressions or strains, and accordingly, the bias cords (c) at the inner peripheral side of the main cords may be broken. When the bias cords (c) are broken, stripping-off of rubber may occur with the broken portion of the bias cords (c) as a start point. Further, chipping of rubber, rusting of main cords resulting from stripping or chipping or the like may occur. Accordingly, life of the crawler may be considerably shortened.